Centralizers are used in the oil, gas or water well drilling industries to centre a casing within a borehole or previously installed tubular.
Such casings are generally constructed in handleable lengths or ‘joints’, e.g. 12 m (40 ft). The assembly of the casings to a predetermined total length is referred to as a ‘string’, which is often a number of casings coupled together by couplers. Centralizers are disposed along the length of the string. Centralizers are comprised of circular end collars between which are affixed a number of bow springs. Stop devices, such as stop collars, are used to maintain centralizers at axial positions along casing joints.
There are ever increasing instances where the annular width between the casing and the Inner Diameter (ID) of the previously installed tubular will not permit passage of centralisers and stop devices. Annular clearances have become especially limited or small with the increase in ‘deep water wells’. Furthermore, annular clearances have become reduced due to the necessity that well designs combat the increased pressures and exhibit the required tensile strengths.
To address this centralizer subs have been developed, integrally fulfilling the purpose of the centralizer, coupler between lengths of casing, and axial stop devices. Centraliser subs are generally made from coupling stock normally conforming to API 5CT, which may be machined to certain required dimensions. The coupling stock used for the centralizer sub bodies, have a variability in their Outer Diameter (OD) due to manufacturing “drift”, but must conform with regulatory requirements such as API 5CT. These requirements ensure that these devices will be fit for purpose, and define and provide tolerances for diameters, and wall thicknesses etc. Centralizer subs are positioned and threaded between casing joints. Existing centralizer subs have various machined recesses on their OD which contain a spring bow centraliser. The purpose of these recesses is to allow full compression of the spring bows to below the surface of the maximum OD of the centralizer sub body. However, there are a number of issues or problems with existing centralizer subs, as now discussed.
The recesses are often machined to a depth that results in a wall thickness of the centralizer sub body that falls below that required for the API 5CT standard. There is then a reduction of the collapse pressure and tensile strength of the centralizer sub compared to the parent casing joints. To mitigate this, the material for the centralizer sub may be selected with higher mechanical strengths than that for the normal coupling stock. However, as a result material costs and material processing costs are increased.
Another existing solution is to provide a centralizer, coupled to the centralizer sub body, made from thinner than normal material. However, this results in a number of problems:    1. The centralizer exhibits a poor ‘restoring force’ when expanded inside an open or under reamed borehole. This may be compensated for by decreasing the length of the bow springs. However, this leads to higher insertion and running forces (drag).    2. Keeping thinner spring bows at their standard length, results in high stress loads at the join between the spring bow and the end bands. When the spring bows are fully compressed, permanent deformation of the spring bows can result. The required standoff, or minimum annulus width between the casing and the existing tubular in the open borehole will then be deficient and not comply with regulatory requirements. However, thinner rather than longer spring bows are typically used.
Additionally, centralisers are affixed in the machined recesses of the centralizer sub body by butt welding the end bands in situ within the recess. This leads to a number of problems:    1. Welds are performed from the outer side of end bands. This results in a poor, weak, weld joint of the thinner low strength material. The centralizer then can have a reduced strength in use, especially with very narrow annuli and high undulations of the well profile;    2. Because the welds are performed from the outside, the caps of the welds sit proud of the surface of the centralizer end bands. This means that one or any of the below are required to be done, with associated resulting problems:            a. the recess in the centralizer sub body has to be made deeper to accommodate the thickness of the weld cap, thereby further compromising the strength of the centralizer sub body or requiring even higher performance materials;        b. the centralizer end bands have to be made even thinner thereby further compromising the strength of the centralizer;        c. the caps of the welds have to be ground down to meet clearance OD demands, thereby weakening the welds still further.        
Therefore, in summary many existing centralizer subs suffer from very poor centraliser manufactured fits with subsequent impact on diameter increases and poor quality welds; the centralizer sub bodies have needed to be machined below the minimum OD tolerance causing concerns relating to the effect upon tensile strength, collapse and burst pressures; and stronger materials have frequently been used to compensate for or mitigate those concerns leading to an increase in cost for those materials and their processing and machining.